SpaceX has added another string to its now weighty bow by successfully test firing the Merlin 1D engine, which will propel future craft into the thermosphere. The 1D is the sequel to the Merlin engines used to convey the DragonX to the International Space Station, with an improved thrust-to-weight ratio that reportedly makes it the most efficient booster engine ever built. It's hoped that the gear will be ready to make the jump to full use in time for the sixth flight of the Falcon 9, currently pencilled in for 2013. If you're the sort who enjoys watching a big pile of fire being pushed into a concrete chamber, you're really gonna love the video after the break.